1. Technical Field
The present invention relates generally to a membrane and method of making the membrane. In particular, the present invention relates to a porous membrane that has a coating to provide oleophobic properties to the membrane and to a method of coating the membrane.
2. Description of the Prior Art
Various known technical fabrics are suitable for use in demanding applications. Examples of such demanding applications include filter elements, outerwear garments, tents, sleeping bags, protective garments, clean room garments, surgical drapes, surgical gowns and other types of barrier wear. The known fabrics often include a film or membrane to protect the fabric user from an external condition or environment and/or protect the external environment from contamination by the user. The film or membrane may be made from any suitable material, structure and manner.
A known material for the membrane that has proven particularly suitable for such demanding applications is made of an expanded polytetrafluoroethylene (ePTFE) material. The ePTFE membrane is typically laminated to at least one suitable material, such as a base or shell fabric. The resulting membrane and fabric laminate can then be used to manufacture any number of finished products to meet the demands of the particular application.
It is known that an ePTFE membrane is air permeable and moisture vapor transmissive, yet resistant to wind and liquid penetration at moderate pressures. However, the ePTFE membrane tends to absorb oils and certain contaminating agents, such as body oils contained in perspiration, fatty substances or detergent-like contaminants. When the ePTFE membrane becomes contaminated by absorbing oils or other contaminating agents, the membrane may no longer effectively resist liquid penetration.
One known approach at rendering an ePTFE membrane resistant to contamination by absorbing oils or contaminating agents includes applying a layer of polyurethane onto, or partially into, the ePTFE membrane, as disclosed in U.S. Pat. No. 4,194,041. A membrane with a polyurethane layer has wash durable oil and contaminating agent resistance and relatively high moisture vapor transmission rates. However, air may not freely permeate through the polyurethane layer. It is known that some degree of air permeability is desirable to increase user comfort.
Another known approach is to coat surfaces defining the pores in the membrane with a fluoroacrylate monomer, as disclosed in U.S. Pat. No. 5,156,780 then polymerize. The monomer is polymerized in situ to coat surfaces defining the pores in the membrane. This approach provides a membrane that is somewhat air permeable and resistant to absorbing oils and contaminating agents. However, this approach requires a polymerization initiator to provide the desired oleophobic properties and a specialized monomer composition. This approach also requires relatively expensive equipment and materials, such as an ultraviolet curing station and a nearly oxygen-free or inert atmosphere, to process and polymerize the monomer once it is applied to the membrane. Furthermore, this approach requires solvents that may be environmentally unsound.
Yet another known approach is to coat a microporous membrane with an organic polymer having recurring pendant fluorinated organic side chains, as disclosed in U.S. Pat. No. 5,539,072. The polymer is applied to the membrane in an aqueous dispersion. The dispersion has a relatively small particle size in the range of 0.01 to 0.10 micron so the particles can enter pores in the membrane. A relatively expensive fluorosurfactant is used in this approach. The fluorosurfactant is used in amounts that may be difficult to completely remove from the membrane.
Thus, a need exists to provide a membrane that is air permeable, moisture vapor transmissive, wind and liquid penetration resistant, durably resists absorbing oils and certain contaminating agents, is relatively inexpensive and easy to manufacture, made from readily available materials and does not require relatively expensive equipment or processes.
The present invention is directed to sheet material that is moisture vapor transmissive, air permeable, wind and liquid penetration resistant and resistant to contamination from absorbing oils and contaminating agents. The sheet material of the present invention can be in the form of numerous structures, for example a laminated fabric including a base or shell fabric laminated to a composite membrane embodying the present invention or just the composite membrane. The present invention is also directed to a method of coating the membrane.
The composite membrane embodying the present invention is relatively inexpensive and easy to manufacture, made from readily available materials and does not require relatively expensive equipment or complicated processes. The composite membrane embodying the present invention, includes a membrane having a structure of nodes connected by fibrils. Surfaces of the nodes and fibrils define a plurality of interconnecting pores extending through the membrane between major sides of the membrane. The membrane is moisture vapor transmissive, air permeable, wind and liquid penetration resistant and made from a material that tends to absorb oils and certain contaminating agents. A coating is disposed on surfaces of the nodes and fibrils that define pores in the membrane. The coating comprises an oleophobic fluoropolymer. The oleophobic fluoropolymer coating is coalesced on surfaces of the nodes and fibrils to provide resistance to oil and contaminating agents without completely blocking the pores in the membrane.
The membrane is preferably made from expanded polytetrafluoroethylene. The acrylic-based polymer with fluorocarbon side chains is preferably a perfluoroalkyl acrylic copolymer. The fluorocarbon side chains extend in a direction away from the surface of the nodes and fibrils that the coalesced oleophobic fluoropolymer coats.
The method of treating a membrane according to the present invention comprises the steps of providing a membrane with surfaces defining a plurality of pores extending through the membrane. A dispersion of an oleophobic fluoropolymer, such as an acrylic-based polymer with fluorocarbon side chains, is provided. The dispersion is diluted with a water-miscible wetting agent. The diluted dispersion wets surfaces that define pores in the membrane. The wetting agent is removed. Oleophobic fluoropolymer solids in the dispersion are coalesced on surfaces of the nodes and fibrils of the membrane to render the membrane resistant to contamination from absorbing oils and contaminating agents without completely blocking the pores.
The step of providing a membrane preferably comprises providing a microporous membrane made from expanded polytetrafluoroethylene. The step of providing a dispersion of an oleophobic fluoropolymer comprises providing a dispersion of acrylic-based polymer with fluorocarbon side chains.
The step of providing a dispersion of acrylic-based polymer with fluorocarbon side chains comprises providing a perfluoroalkyl acrylic copolymer. The step of providing a perfluoroalkyl acrylic copolymer comprises providing a water-miscible dispersion of perfluoroalkyl acrylic copolymer solids in water-miscible solvent. The coalescing step comprises heating the treated membrane.
The diluting step comprises diluting the dispersion of oleophobic fluoropolymer with a water-miscible wetting agent. The diluting step comprises diluting the dispersion at a ratio of water-miscible wetting agent to dispersion in a range of about 1:5 to 20:1. The diluted dispersion has surface tension and relative contact angle properties that enable the diluted dispersion to wet the membrane and coat surfaces defining the pores in the membrane. The diluting step further includes diluting the dispersion in a material selected from the group including ethanol, isopropyl alcohol, methanol, n-propanol, n-butanol, N-N-dimethylformamide, methyl ethyl ketone and water soluble e- and p- series glycol ethers.